The present invention relates to a flow device and method, and more particularly to a flow device and method for lowering the exit velocity of a gaseous medium flowing through the device by shifting the location of shock upstream from where it would otherwise occur when the pressure ratio across the device is high.
U.S. Pat. No. 3,778,038 granted Dec. 11, 1973, explains a method and apparatus for producing a uniform combustible mixture of air and minute liquid fuel droplets for delivery to the intake manifold of an internal combustion engine. This apparatus includes an intake air zone connected to a variable area throat zone for constricting the flow of air to increase its velocity to sonic. Liquid fuel is introduced into the air stream to minutely divide and uniformly entrain fuel as droplets in the air flowing through the throat zone. Wall structure downstream from the throat zone is arranged to provide a gradually diverging zone for efficiently recovering a substantial portion of the kinetic energy of the high velocity air and fuel mixture as static pressure. Such efficient conversion enables the maintenance of sonic velocity air and fuel through the throat zone over substantially the entire operating range of the engine to which the air and fuel mixture is supplied.
As further explained in the above U.S. patent, during flow conditions when the pressure ratio across the apparatus is high, supersonic flow occurs and a shock results downstream from the throat zone. As this pressure ratio increases, the shock moves down the gradually diverging zone and further away from the throat zone. With even higher pressure ratios across the apparatus, the shock moves further down the gradually diverging zone. After shock under any conditions there is a tendency for the flow to separate from the walls of the gradually diverging zone. Usually the flow simply reattaches to the walls but when the shock is far down the gradually diverging zone, there is not sufficient wall space for such reattachment and an excessively high velocity jet is formed at the point of discharge from the apparatus.
The above U.S. patent also discloses that the device functions to control the mass flow of air being supplied to the engine since the air flow is maintained at sonic velocity through the throat zone over a wide range of engine conditions. Hence, under unvarying atmospheric conditions the mass flow rate of air being supplied to the engine is directly proportional to the cross-sectional area of the throat zone. Finally, as is apparent from the above U.S. patent, the liquid delivery means may be eliminated and the device used solely as a mass flow control for air or any gaseous medium.
The particular divergence of the wall structure downstream from the throat zone is extremely important in order to efficiently recover the kinetic energy of the high velocity mass as static pressure. As explained above, such efficient energy recovery enables sonic velocity at the throat zone over a wide range of downstream pressure conditions. However, the gradually diverging zone formed by the wall structure may be such that the exit velocity of the mass is excessive under the conditions mentioned above when the pressure ratio across the apparatus is high thereby causing shock to occur far down the gradually diverging zone. Then the flow does not reattach to the walls and a high velocity jet emerges from the apparatus. In carburetor applications, for example, excessive exit velocity from the air and fuel mixing device may cause the air and fuel mixture to impinge upon the manifold floor which prevents the mixture from being delivered to the cylinders of the engine in a homogeneous state.